LED downlight fittings or downlighters are a form of lighting unit becoming more and more widely used as light sources in domestic and commercial environments. They offer significant energy savings when compared with traditional incandescent lighting, whilst being particularly neat and unobtrusive in their appearance, since almost the entire downlight fitting is concealed behind a ceiling or other suitable panel or surface, whilst giving out a pleasing light. However, LED downlights suffer from a number of disadvantages.
LED's generate significant amounts of heat. It is important to prevent overheating of the LEDs, and associated control circuit, since overheating will have obvious detrimental effects on the light output and service life of these components. Indeed, excessive temperatures will cause LEDs and electronic components to fail leading to premature failure of the lighting unit. To this end it is known to provide LED lighting units with cooling means in the form of a heat sink or a cooling fan to draw heat away from the lighting element to the rear of the lighting unit.
Additionally, their installation generally requires an aperture to be cut in a ceiling or other surface and that surface can be required by relevant Building Regulations to act as a fire barrier for typically between 30 minutes to 90 minutes. Downlights are generally installed into an aperture in the ceiling that has to be relatively wide to accommodate the downlight assembly and this thereby compromises the ability of the ceiling to contain a fire in a room below.
A fire rating is the ability to withstand a specified temperature for a period of time without failure. By way of example in UK building regulations, the specified temperature is around 1100 degrees C., but this will vary from country to country and test protocol to test protocol. It is therefore generally considered that materials with a melting point greater than 1000 degrees C. will withstand such fire tests, whereas downlights made from materials having a melting point less than 1000 degrees C. will not withstand the test conditions for 90 minutes. Thus a downlight made partly from aluminium would inevitably fail such a test.
However, there is a desire to use a solid state lighting element in fire rated fixtures, where the solid state lighting element is able efficiently to transfer heat to a heat sink, but this creates a problem in that many such LEDs are mounted on aluminium circuit boards which in turn are directly connected to aluminium heat sinks, and thus the combination of LED circuit board and heat sink has a melting point well below the requirement needed to achieve the required fire rating. In the case of aluminium the melting point is only in the region of 660 degrees C.
It is an advantage of the present invention that it allows for the use of a solid state lighting element/heat sink combination having a melting point lower than the temperature applied during fire rating testing, while allowing the downlight fixture to pass the fire rating test.